This invention is concerned particularly with the detection of foreign bodies in tobacco used to manufacture cigarettes. It is common practice to provide a magnet arrangement for detecting and removing ferrous metal foreign bodies in cigarette machine hoppers. Non-ferrous metallic particles can be detected in the finished cigarettes, if necessary, in various ways, for example by means an infra-red or other beam of energy which is interrupted by such foreign bodies. It has, however, generally been accepted that non-metallic foreign bodies, such as pieces broken off conveyor bands, cannot readily be detected since they are transparent to infra-red beams or to beams of other frequencies which could be used, on the other hand, it is desirable to be able to detect such foreign bodies as their presence in the finished cigarettes is objectionable and could in some instances constitute a health hazard to the smoker.